Monoamine neurotransmitters (dopamine, norepinephrine, and serotonin) play important roles in modulating the strength of both excitatory and inhibitory neurotransmission via activation of the corresponding receptors. Monoamine neurons project their axons throughout the brain and regulate diverse brain functions including arousal, stress, emotion, reward, learning, and cognition. On a cellular level, monoamines modulate responsiveness of the target cells and synapses. Aberrations in monoamine neurotransmission have been implicated in numerous neurological and neuropsychiatric disorders including Parkinson's disease, schizophrenia, ADHD, drug addiction, depression, and anxiety. However, due to methodological limitations, monoamine neurotransmission have only been studied on the bulk level of large ensembles of monoaminergic synapses. There have been no experimental tools to examine the monoamine release characteristics of individual presynaptic boutons, a fundamental physiological parameter. Drs. Dalibor Sames (PI-1) and David Sulzer (PI-2) established an interdisciplinary research program focused on addressing this challenge. Specifically, optical probes termed Fluorescent False Neurotransmitters (FFNs) were designed as tracers of dopamine. This new application aims to expand the scope of the FFN concept to the entire monoamine system, many different brain areas, and living rodents. We propose to develop FFN probes selective for norepinephrine and serotonin synapses, and in vivo microscopic imaging methods applicable in living rodents. The proposed new imaging methods will enable, for the first time, visualization of synaptic content release at individual presynaptic terminals of specific neurochemical types in vivo in several brain areas (striatum, somatosensory cortex, hippocampus). These new probes and associated imaging methods will unlock the possibility of addressing many long-standing questions about release properties of single synapses and their physiological regulation and deregulation in rodent disease models. The FFN probes are compatible with the existing calcium and voltage sensors and thus jointly these tools will provide a more complete readout of synaptic function and plasticity in intact circuitry.